1. Field of the Invention
The present invention relates to a display device or a semiconductor device. In specific, the present invention relates to a hold display device such as a liquid crystal display device. Further, the present invention relates to a method for driving a liquid crystal display device in which the light-emission luminance of a backlight is partly controlled. Furthermore, the present invention relates to an electronic device including the display device in its display portion.
2. Description of the Related Art
Liquid crystal display devices can be formed thinner and more lightly than display devices formed using cathode ray tubes (CRT). Further, the liquid crystal display devices have advantages such as low power consumption. Furthermore, as the liquid crystal display device, a variety of liquid crystal display devices can be used, for example, from small liquid crystal display devices whose diagonal sizes of display portions are several inches to large liquid crystal display devices whose diagonal sizes of display portions are 100 inches. Therefore, the liquid crystal display devices are widely used as display devices of a variety of electronic devices such as cell phones, still cameras, video cameras, and television receivers.
In recent years, thin display devices including liquid crystal display devices have been widely used; however, their image quality is not always satisfactory. Therefore, approach to improve image quality is still continued. For example, problems of the image quality of the liquid crystal display devices include decrease in image quality (contrast ratio or color reproducibility) due to leakage of light from a backlight, decrease in the quality of moving images due to generation of afterimages caused by a hold display device (or a hold drive display device), or the like. Note that the hold display device is a display device in which luminance hardly changes and is maintained during one frame period. On the other hand, like a CRT, a display device in which display is performed by emitting light only in a very short time during one frame period is referred to as an impulsive display device (or an impulsive driving display device).
As a technology component for improving the quality of images displayed by a liquid crystal display device, a technique for controlling the quality of images by partly changing the tight-emission luminance of a backlight has been known. This technique improves image quality by partly decreasing light from the backlight in a portion in which display is dark on a screen to suppress leakage of light from the backlight. As a technique for achieving such display, for example, Patent Document 1 and Patent Document 2 are published.